The overall aim of this project is to understand the neural mechanisms of sound localization and selective auditory attention. These results will help to understand how the brain integrates auditory information from the two ears and how it can direct attention to one source over others in a noisy environment. This circuitry plays a major role in our ability to detect signals in the presence of background noise, which is the major symptom of elderly people with hearing loss. Understanding the neural mechanisms will help in the design of hearing aids and therapy. There are two major aims. First, we will use two well-known spatial auditory illusions to study the representation of sound sources in space in the inferior colliculus. Experiments will use a combined behavioral and physiological preparation in awake and trained cats. We will continue our on-going experiments on the psychophysics and physiology of the precedence effect and begin a new series of experiments on the Franssen effect. Both of these are robust auditory illusions that can be readily demonstrated in our behaving preparation. Second, we will develop a new psychophysical paradigm for studying selective auditory attention. Cats will be trained to attend to a speaker on one side in order to make frequency discriminations while ignoring sounds from the contralateral side. We hypothesize that attending to one side will enhance the ability to perform the discrimination as compared to trials in which the animal is not cued to attend to one side or the other. We will then make physiological recordings from the auditory cortex using an innovative silicone multichannel probe to look for modulation by attention during this task. Spatial hearing and selective attention are important basic functions of the auditory system: defects in binaural function in human patients can lead to considerable difficulty in understanding conversations in a noisy room, which is the most common complaint of the hearing-impaired and can lead to severe social withdrawal.